Transcript Chapter 24. Amines

John E. McMurry
www.cengage.com/chemistry/mcmurry
Chapter 24
Amines and Heterocycles
Paul D. Adams • University of Arkansas
Amines – Organic Nitrogen
Compounds

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Organic derivatives of ammonia, NH3
Nitrogen atom with a lone pair of electrons, making
amines both basic and nucleophilic
Occur in plants and animals
Why this Chapter?

Amines and carbonyl compounds are the most
abundant and have rich chemistry

In addition to proteins and nucleic acids, a
majority of pharmaceutical agents contain amine
functional groups
24.1 Naming Amines


Alkyl-substituted (alkylamines) or aryl-substituted
(arylamines)
Classified: 1° (RNH2), methyl (CH3NH2), 2°
(R2NH), 3° (R3N)
Quaternary Ammonium Ions


A nitrogen atom with four attached groups is
positively charged
Compounds are quaternary ammonium salts
IUPAC Names – Simple
Amines

For simple amines, the suffix -amine is added to
the name of the alkyl substituent
IUPAC Names – “-amine”
Suffix

The suffix -amine can be used in place of the
final -e in the name of the parent compound
IUPAC Names – Amines with
More Than One Functional Group

Consider the –NH2 as an amino substituent on
the parent molecule
IUPAC Names – Multiple Alkyl
Groups

Symmetrical secondary and tertiary amines are
named by adding the prefix di- or tri- to the alkyl
group
IUPAC Names – Multiple,
Different Alkyl Groups


Named as N-substituted primary amines
Largest alkyl group is the parent name, and other
alkyl groups are considered N-substituents
Common Names of Heterocyclic
Amines
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If the nitrogen atom occurs as part of a ring, the
compound is designated as being heterocyclic
Each ring system has its own parent name
24.2 Properties of Amines

Bonding to N is similar to that in ammonia
 N is sp3-hybridized
 C–N–C bond angles are close to 109° tetrahedral
value
Chirality Is Possible (But Not
Observed)


An amine with three different substituents on nitrogen is
chiral (in principle but not in practice) : the lone pair of
electrons is the fourth substituent
Most amines that have 3 different substituents on N
cannot be resolved because the molecules interconvert
by pyramidal inversion
Amines Form H-Bonds


Amines with fewer than five carbons are water-soluble
Primary and secondary amines form hydrogen bonds,
increasing their boiling points
24.3 Basicity of Amines
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The lone pair of electrons on nitrogen makes amines
basic and nucleophilic
They react with acids to form acid–base salts and they
react with electrophiles
Relative Basicity

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Amines are stronger
bases than alcohols,
ethers, or water
Amines establish an
equilibrium with water in
which the amine
becomes protonated and
hydroxide is produced
The most convenient way
to measure the basicity
of an amine (RNH2) is to
look at the acidity of the
corresponding
ammonium ion (RNH3+)
High pKa → weaker acid
and stronger conjugate
base.
General Patterns of Basicity



Table 24.1: pKa values of ammonium ions
Most simple alkylammmonium ions have pKa's of
10 to 11
Arylamines and heterocyclic aromatic amines are
considerably less basic than alkylamines
(conjugate acid pKa 5 or less)
Amides



Amides (RCONH2) in general are not proton acceptors except
in very strong acid
The C=O group is strongly electron-withdrawing, making the N
a very weak base
Addition of a proton occurs on O but this destroys the double
bond character of C=O as a requirement of stabilization by N=
24.4 Basicity of Arylamines

The N lone-pair electrons in arylamines are delocalized
by interaction with the aromatic ring  electron system
and are less able to accept H+ than are alkylamines
Substituted Arylamines
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Can be more basic or less basic than aniline
Electron-donating substituents (such as –CH3,
–NH2, –OCH3) increase the basicity of the
corresponding arylamine
Electron-withdrawing substituents (such as –Cl,
–NO2, –CN) decrease arylamine basicity
Substituted Arylamines
(Cont’d)
24.5 Biological Amines and the
Henderson-Hasselbalch Equation
Henderson-Hasselbalch Equation:
A 
pH  p K a  log  
 HA 

so
A 
log    pH  p K a
 HA 
In what form do amines exist at physiological pH inside
cells?
24.6 Synthesis of Amines
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Arylamines are
prepared from nitration
of an aromatic
compound and
reduction of the nitro
group
Reduction by catalytic
hydrogenation over
platinum is suitable if
no other groups can
be reduced
Iron, zinc, tin, and
tin(II) chloride are
effective in acidic
solution
SN2 Reactions of Alkyl Halides

Ammonia and other amines are good
nucleophiles
Uncontrolled Multiple
Alkylation

Primary, secondary, and tertiary amines all have
similar reactivity, the initially formed
monoalkylated substance undergoes further
reaction to yield a mixture of products
Selective Preparation of Primary
Amines: The Azide Synthesis
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Azide ion, N3 displaces a halide ion from a primary or
secondary alkyl halide to give an alkyl azide, RN3
Alkyl azides are not nucleophilic (but they are explosive)
Reduction gives the primary amine
Gabriel Synthesis of Primary
Amines

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A phthalimide alkylation for preparing a primary amine
from an alkyl halide
The N-H in imides (–CONHCO–) can be removed by
KOH followed by alkylation and hydrolysis
Reductive Amination of Aldehydes
and Ketones

Treatment of an aldehyde or ketone with
ammonia or an amine in the presence of a
reducing agent
Reductive Amination Is
Versatile

Ammonia, primary amines, and secondary amines yield
primary, secondary, and tertiary amines, respectively
Mechanism of Reductive
Amination
Reducing Step


Sodium cyanoborohydride, NaBH3CN, reduces
C=N but not C=O
Stable in water
Hofmann and Curtius
Rearrangements
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Carboxylic acid derivatives can be converted into
primary amines with loss of one carbon atom by
both the Hofmann rearrangement and the Curtius
rearrangement
Hofmann Rearrangement

RCONH2 reacts with
Br2 and base

Gives high yields of
arylamines and
alkylamines
Curtius Rearrangement


Heating an acyl azide prepared from an acid
chloride
Migration of –R from C=O to the neighboring
nitrogen with simultaneous loss of a leaving
group
24.7 Reactions of Amines

Alkylation and acylation have already been
presented
Hofmann Elimination
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Converts amines into alkenes
NH2 is very a poor leaving group so it is
converted to an alkylammonium ion, which is a
good leaving group
Silver Oxide Is Used for the
Elimination Step

Exchanges hydroxide ion for iodide ion in the
quaternary ammonium salt, thus providing the
base necessary to cause elimination
Orientation in Hofmann
Elimination

We would expect that the more highly
substituted alkene product predominates in the
E2 reaction of an alkyl halide (Zaitsev's rule)

However, the less highly substituted alkene
predominates in the Hofmann elimination due to
the large size of the trialkylamine leaving group

The base must abstract a hydrogen from the
most sterically accessible, least hindered
position
Steric Effects Control the
Orientation
24.8 Reactions of Arylamines
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Amino substituents are strongly activating, ortho- and
para-directing groups in electrophilic aromatic substitution
reactions
Reactions are controlled by conversion to amide
Arylamines Are Not Useful for
Friedel-Crafts Reactions


The amino group forms a Lewis acid–base complex with
the AlCl3 catalyst, preventing further reaction
Therefore we use the corresponding amide
Diazonium Salts: The Sandmeyer
Reaction

Primary arylamines react with HNO2, yielding
stable arenediazonium salts
Uses of Arenediazonium Salts

The N2 group can be replaced by a nucleophile
Preparation of Aryl Halides
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Reaction of an arenediazonium salt with CuCl or CuBr
gives aryl halides (Sandmeyer Reaction)
Aryl iodides form from reaction with NaI without a
copper(I) salt
Aryl Nitriles and Carboxylic
Acids

An arenediazonium salt and CuCN yield the
nitrile, ArCN, which can be hydrolyzed to
ArCOOH
Formation of Phenols (ArOH)

From reaction of the arenediazonium salt with
copper(I) oxide in an aqueous solution of
copper(II) nitrate
Reduction to a Hydrocarbon

By treatment of a diazonium salt with
hypophosphorous acid, H3PO2
Mechanism of Diazonium
Replacement

Through radical (rather than polar or ionic)
pathways
Diazonium Coupling Reactions

Arenediazonium salts undergo a coupling
reaction with activated aromatic rings, such as
phenols and arylamines, to yield brightly colored
azo compounds, Ar–N=N–Ar
How Diazonium Coupling
Occurs
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The electrophilic diazonium ion reacts with the electronrich ring of a phenol or arylamine
Usually occurs at the para position but goes ortho if para
is blocked
Azo Dyes
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Azo-coupled products have extended  conjugation that
lead to low energy electronic transitions that occur in
visible light (dyes)
24.9 Heterocyclic Amines
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A heterocycle is a cyclic compound that contains
atoms of two or more elements in its ring, usually
C along with N, O, or S
Pyrole and Imidazole
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Pyrole is an amine and a conjugated diene,
however its chemical properties are not
consistent with either of these structural features
Chemistry of Pyrole
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Electrophilic substitution reactions occur at C2 because it
is position next to the N

A more stable intermediate cation having 3 resonance
forms
At C3, only 2 resonance forms

Polycyclic Heterocycles
24.10 Spectroscopy of Amines Infrared
Infrared Spectroscopy:
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Characteristic N–H stretching absorptions 3300 to
3500 cm1
Amine absorption bands are sharper and less intense
than hydroxyl bands
 Protonated amines show an ammonium band in
the range 2200 to 3000 cm1
Examples of Infrared Spectra
Nuclear Magnetic Resonance
Spectroscopy


N–H hydrogens appear as broad signals without
clear-cut coupling to neighboring C–H hydrogens
In D2O exchange of N–D for N–H occurs, and the
N–H signal disappears
Chemical Shift Effects
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Hydrogens on C next to N absorb at lower field
than alkane hydrogens
N-CH3 gives a sharp three-H singlet at  2.2 to 
2.6
13C

NMR
Carbons next to amine N are slightly deshielded about 20 ppm downfield from where they would
absorb in an alkane
Mass Spectrometry
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A compound with an odd number of nitrogen
atoms has an odd-numbered molecular weight
and a corresponding parent ion
Alkylamines cleave at the C–C bond nearest the
nitrogen to yield an alkyl radical and a nitrogencontaining cation
Mass Spectrum of
Ethylpropylamine

The two main modes of a cleavage give fragment ions at
m/z = 58 and m/z = 72.
Let’s Work a Problem
What are the major products that you might expect
from Hofmann elimination of the following amine?
Answer
The hydrogens that can be eliminated are
covalently attached to the carbon bound to the
Nitrogen. In this case, there are 2 such places
where a hydrogen could be eliminated to give the
cyclopentene